Though present in natural settings at minute quantities, ethylene oxide was first synthesized in a laboratory setting in 1859 by Alsatian chemist Charles-Adolphe Wurtz using the so-called “chlorohydrin” process. However, the usefulness of ethylene oxide as an industrial chemical was not fully understood in Wurtz's time; and so industrial production of ethylene oxide using the chlorohydrin process did not begin until the eve of the First World War due at least in part to the rapid increase in demand for ethylene glycol (of which ethylene oxide is an intermediate) as an antifreeze for use in the rapidly growing automobile market. Even then, the chlorohydrin process produced ethylene oxide in relatively small quantities and was highly uneconomical.
The chlorohydrin process was eventually supplanted by another process, the direct catalytic oxidation of ethylene with oxygen, the result of a second breakthrough in ethylene oxide synthesis, discovered in 1931 by French chemist Thèodore Lefort. Lefort used a solid silver catalyst with a gas phase feed that included ethylene and utilized air as a source of oxygen.
In the eighty years since the development of the direct oxidation method, the production of ethylene oxide has increased so significantly that today it is one of the largest volume products of the chemicals industry, accounting, by some estimates, for as much as half of the total value of organic chemicals produced by heterogeneous oxidation. Worldwide production in the year 2000 was about 15 billion tons. (About two thirds of the ethylene oxide produced is further processed into ethylene glycol, while about ten percent of manufactured ethylene oxide is used directly in applications such as vapor sterilization.)
The growth in the production of ethylene oxide has been accompanied by continued innovation in catalysis and processing. Recently, and of particular interest to practitioners in the ethylene oxide field has been alternative sources of ethylene feedstock. Conventionally, ethylene is derived from petroleum sources, especially naptha, by thermal cracking with steam. This interest in alternative feedstocks is a result not only of the sustained increase in the price of crude oil but also because of greater environmental consciousness of the importance of using renewable and abundant hydrocarbon sources.
One such renewable hydrocarbon source that has attracted considerable recent interest is bioethanol-derived ethylene. Bioethanol itself is obtained by fermentation of vegetable biomass and agricultural byproducts and wastes—and thus is abundant and renewable. The fermentation of biomass to ethanol results in mixtures containing about 95% water and 5% ethanol. The water is then separated out using a combination of azeotropic distillation or solvent extraction. To produce ethylene the ethanol is then sent to a dehydration process where it is reacted over a dehydration catalyst to from ethylene, which then forms a primary feedstock or feed component for ethylene oxide or one or more ethylene oxide-derivatives. This return to bioethanol for producing ethylene is ironic because when ethylene was first synthesized in the middle of the 19th century, it was obtained by dehydrating ethanol in the presence of a homogeneous phosphorus catalyst. (Roscoe, H. A. and Schorlemmer, C., A Treatise on Chemistry, 1878, 612).
While bioethanol-derived ethylene offers the advantage of being an alternative feedstock, abundant and renewable and semi-independent from the world's petroleum market, it also presents certain challenges. Most notably there is the problem that despite attempts to remove impurities and separate byproducts from the bioethanol-derived ethylene, certain contaminants remain that must be treated and removed. For example sulfur-containing compounds, not only the commonly found and relatively-easy-to-remove hydrogen sulfide, but more recalcitrant sulfur-containing compounds such as the refractory organic sulfurs which include, mercaptans, thiophenes, and carbonyl sulfide, are frequently found in bioethanol-derived ethylene. In fact, the presence of sulfur as a byproduct of ethanol dehydration was identified in Roscoe and Schlorlemmer's analytical protocol of the dehydration process—which even specified the use of a caustic scrubbing wash to eliminate it.
As a contaminant sulfur has long been identified as a particularly serious catalyst poison. This is particularly the case for the adsorption of sulfur onto silver, in fact the affinity of silver for sulfur can be seen as the tarnish that visibly forms on silver objects which absorb hydrogen sulfide and other sulfur compounds from the ambient air to form a layer of sulfides. Sulfur is particularly pernicious in an ethylene oxide system as it has long been known as severely and irreversibly poisonous to Ag-based ethylene oxide catalysts (Rebsdat, S. and Mayer D., 2005, Ethylene Oxide, Ullmans Encyclopedia of Industrial Chemistry).
A variety of techniques are available to treat hydrocarbon streams containing sulfur-compounds. In adsorptive desulfurization, which is perhaps the easiest and mostly widely used desulfurization technique, the hydrocarbon stream is passed through an adsorbent guard bed to adsorb the sulfur-containing compounds by physical and/or chemical adsorption processes. Most typically, the adsorbent comprises a granular inorganic material such an inorganic oxide; typical examples include zinc oxide, copper oxide, and aluminum oxide but may also be selected from other transition metal oxides and rare earth metal oxides. Preferably guard bed materials are chosen based on their selectivity to adsorbing certain sulfur species, for example copper oxide or zinc oxide are effective at removing simple sulfur compounds, like hydrogen sulfide; while alumina-based adsorbents have some capability for adsorbing organic sulfurs, against which other adsorbing metal oxides are completely ineffectual.
However, while aluminum oxide may be better than zinc oxide for removing organic sulfurs, for many applications it is simply not efficient. Accordingly, other desulfurization techniques, such as hydrodesulfurization may be used instead. In hydrodesulfurization (HDS), a hydrocarbon stream is reacted with hydrogen gas at high temperatures and high pressures over a hydrogenation catalyst. HDS is more effective at removing organic sulfurs than metal oxide adsorbent beds, but it still fails to remove some organic sulfurs. While HDS is more effective than other desulfurization techniques, readily converting mercaptans and thioethers, it fails to convert other organic sulfurs such as substituted and unsubstituted thiophenes. Moreover, HDS is costly and requires high temperatures and pressures.
One alternative to hydrodesulfurization is Oxidative Desulfurization (“ODS”). In ODS a refractory organic sulfur-containing hydrocarbon is contacted with a strong oxidant (such as hydrogen peroxide, an organic peroxide, or organic peracid) in the presence of a metal catalyst, typically one such as titanium, zirconium, chromium, tungsten and molybdenum, to form an organosulfone, which can then be removed by distillation or by further chemical reaction. Compared to hydrodesulfurization, ODS has the advantage of not requiring high temperatures or pressures for operation and ODS removes a fuller spectrum of organic sulfurs than hydrodesulfurization. Thus, ODS has a wider spectrum of refractory organic sulfurs to which it will apply and has less demanding temperature and pressure requirements for reducing the sulfur content in the feedstock sources. However, like HDS, ODS has the disadvantage of being considerably more costly and complicated to implement than a guard bed. The additional complexity not only increases costs but also reduces process flexibility and operability. Additionally ODS requires the provisioning of strong oxidizing agents which are expensive and burdensome to handle.
Given the foregoing there is a continuing need in the art for a desulfurization process that effectively removes a wider spectrum of refractory organic sulfurs than conventional inorganic materials, and yet at the same time can be operated more conveniently with more flexibility and less expensive than conventional hydrodesulfurization and oxidative desulfurization techniques.